Storm Forecast
Valid: Sat 08 Jul 2017 06:00 to Sun 09 Jul 2017 06:00 UTC
Issued: Sat 08 Jul 2017 06:02
Forecaster: TASZAREK
A level 1 was issued for parts of SW and CNTRL France mainly for large hail, severe wind gusts, excessive precipitation and tornadoes.
A level 1 was issued for a corridor extending from NE Spain up to Austria mainly mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for a corridor extending from Poland to CNTRL Romania mainly for mainly for large hail, severe wind gusts and excessive precipitation.
SYNOPSIS
A strong westerly mid-level flow dominates in the corridor from British Isles to Ukraine. A cut-off over Spain travels NEwardly into France. Ridge covers most of the CNTRL and E Mediterranean. Two large lows within cold polar airmass are located over Iceland and NE Russia. High theta-e overspreads Europe in the zone from Spain, through France, S CNTRL Europe, Italy, up to Balkan Peninsula. Highest shear values overlapping with unstable airmass cover Poland, W Ukraine and Romania.
DISCUSSION
...NE Spain, France...
Thanks to the cuf-off placed over Spain, a tongue of a well developed warm sector with highly unstable tropical airmass reaches SW France. Under EML and LL moisture convergence NWP models predict ML CAPE up to 2500-3000 J/kg, but considerably capped. Shear is not very high and oscillates around 10-15 m/s. Much higher are values of SRH (200-300 m2/s2) which thanks to the approaching low, provide veering of winds in vertical. Currently it is highly unlikely whether surface-based storms will initiate in the best environment (SW-CNTRL France), but if they do, isolated cells may quickly evolve into supercells capable of producing large hail and severe wind gusts. Enhanced LLS and decreasing LCLs towards evening hours create also a threat for tornado event, especially if discreet convective mode will take place. Lvl. 2 is not issued mainly due to strong uncertainties regarding CI. Much higher QG-lift is expected within approaching trough over NE Spain and extreme SW France. Storms developing in this area will most likely cluster into multicells and given PW exceeding 40mm, will pose a threat of excessive precipitation with possible local flash flooding. MCS in the late evening hours is not ruled out within these storms.
...S Germany, Alps...
This area lacks any substantial large-scale feature to provide strong synoptic-scale lift. However, along with intensifying diurnal heating, evapotranspiration and steep lapse rates, NWP mesoscale models predict a widespread CI in the afternoon hours over S Germany and W Austria. GFS is more positive and includes whole Alpine area in the zone of locally lowered pressure producing widespread convective precipitation. Small to locally moderate instability (> 1500 J/kg ML CAPE) and low shear will favour slowly moving cells capable of producing heavy precipitation, large hail up to 3-4km and downburst-type severe wind gusts. Thanks to wind field interacting with mountains, a local enhancement in vertical shear and helicity is possible, and thus supercell thunderstorms cannot be ruled out. Convection should fade in the nighttime hours along with disappearing instability.
...Poland, W Ukraine, E Slovakia, CNTRL Romania...
Along with WAA this area remains under mid-level jet, which enhances shear values to 20-25 m/s (colocated mainly for DLS). Poor lapse rates and relatively rich boundary layer's moisture provide rather small ML CAPE up to 600-1000 J/kg. Depending on the convective mode, convective updrafts in such environment will evolve most likely into isolated supercells and/or multicell clusters capable of producing large hail, severe wind and excessive precipitation (given PW > 30mm). Thanks to the passage of the weakly pronounced shortwave trough and decent PVA over CNTRL and N Poland, storms should initiate along with intensifying diurnal heating in the afternoon hours in the axis from N Poland, to CNTRL Romania, and move SEwardly. Convection should fade in the late afternoon hours along with a sharp drop in thermodynamic instability. Nighttime elevated convection may however maintain over Baltic countries and W Ukraine.
